1. Field of the Invention
The present invention relates to an etching method for an object to be processed such as a semiconductor wafer, and more specifically to an etching method including a post-etching step for eliminating impurities created in an etching step, method of post-etching process and etching equipment.
2. Description of the Related Art
The degree of integration of the integrated circuit, which is the nucleus of the microelectronics, is presently increasing, and in accordance with an increase in the degree of integration, the width of the pattern is reduced, and the depth thereof is increased. In order to comply with this, there has been developed a dry etching method under a high vacuum condition as a thin film processing technique. In the dry etching, plasma is generated by using a reaction gas in a vacuum, and various materials on a semiconductor substrate is etched by ions, neutral radicals, atoms, and molecules in the plasma. In the dry etching, various types of gas can be used, and a type of gas is seized in accordance with a material to be etched. Thus, selection of an etching gas is a very important factor of the dry etching.
In the case where the material of an object to be processed is, for example, silicon (Si), or silicon oxide (SiO.sub.2), a fluorine-based (F) or fluorine-hydrogen-based (F--H) etching gas is used. In general, carbon tetrafluoride (CH.sub.4) is most commonly used. This is because it is essential in etching of a silicon oxide film that carbon (C) should be present and a great amount of fluorine ion should be generated.
In the etching of a SiO.sub.2 film, the most important factor is to raise the selection ratio of etching with respect to its underlying silicon. In order to increase such selection ratio, there has been proposed an addition of hydrogen to an etching gas, and more specifically, trifluoromethane (CHF.sub.3) is used as such an etching gas. By use of CHF.sub.3, a deposition layer is formed as a cover on the surface of the silicon layer such that the surface of the silicon layer cannot be directly attacked by ions. Consequently, the silicon substrate (wafer) is hardly etched. Also, when attacked by accelerated fluorine ions, the silicon oxide film generates oxygen, by which the polymer formed of C--H on the film can be removed, thereby achieving a sufficient etching process.
During the etching process, a damage layer containing impurities is formed on the bottom of an etching hole. Such a damage layer is removed after the etching process.
The use of CHF.sub.3 as the etching gas makes it likely that polymers adhere to the surface of the etched object. Due to the presence of the polymers, it requires a great amount of time in following steps such as ashing and light etching, after completion of the etching process, and the reproducibility of the etching is degraded.
Further, the polymers and resist include the following drawback. That is, in the case where fluorine radicals are used for removing the damage layer after the etching process, some of the fluorine radicals chemically react with the resist, and the damage layer cannot be fully removed as a result.
In order to solve such a problem, there has been proposed a technique for removing a damage layer, wherein polymers and a resist are removed, first, after the etching process, the object is once placed back to atmosphere, and once again, under a vacuum condition, the damage layer is removed. However, such a technique involves not only a large amount of time, but also a great number of facilities.
Presently, the size of the semiconductor wafer, the object to be processed, is changing from 6 inches to 8 inches, and under such circumstances, the drawbacks described above are delaying the processing time.